The present invention relates to a magnetostatic wave device using magnetic spin resonance of a magnetic thin film formed on a non-magnetic substrate, and in particular to a thin film magnetostatic wave device having an optimum impedance with respect to a high-frequency circuit.
Magnetostatic wave devices have been proposed as devices used for high-frequency oscillation circuits, bandpass filters and the like. The above described magnetostatic wave device is produced by processing a YIG (yttrium iron garnet) thin film, which is formed on a GGG (gadolinium gallium garnet) non-magnetic substrate by the liquid phase epitaxial growth, so as to attain a required shape, for example. (Refer to JP-A-62-245704, for example.)
FIG. 2 is a diagram schematically illustrating the structure of a conventional magnetostatic wave device 1. With reference to FIG. 2, the magnetostatic wave device 1 comprises a magnetostatic wave resonator 6 and a lower conductor plate 7 so disposed as to come in contact with the lower face of a GGG substrate 2 of the magnetostatic wave resonator. The magnetostatic wave resonator 6 comprises a YIG thin film 3 formed on the GGG substrate 2 by using the liquid phase epitaxial method, a plurality of electrode fingers 5 each comprising a metal film, for example an Au film or an Al film formed on this YIG thin film by using the photolithography technique, and pad electrodes 4a and 4b respectively formed on both sides of these electrode fingers 5 by using the photolithography technique. The pad electrode 4b is connected to the lower conductor plate 7 by a wire Wb.
If a bias magnetic field Ho is applied to this magnetostatic wave device 1 in the lengthwise direction of the stripe electrode fingers 5 of the magnetostatic wave resonator 6 by a magnet which is not illustrated, the magnetostatic surface wave propagates on the face of the YIG film 3 and on the boundary face between the YIG thin film and the GGG substrate 2 in a direction, which is defined with respect to Ho by the right-handed screw rule. The magnetostatic surface wave is rotated in the clockwise direction from the surface to the boundary face at the right end portion and from the boundary face to the surface at the left end portion. Resonance is caused at such a frequency that the whole rotation length corresponds to integer times the wavelength. By connecting input and output wires Wa1 and Wa2 of the magnetostatic wave device respectively to terminals a and b of a negative resistance circuit as shown in FIG. 3, for example, therefore, a high-frequency oscillation circuit can be formed. (Refer to JP-A-63-228802.)
FIG. 3 shows a high-frequency oscillation circuit of Colpitts type comprising the magnetostatic wave device 1. Numeral 11 denotes a DC input, and numeral 12 denotes a high-frequency output.
Further, in this circuit, a capacitor 9 prevents the collector of a transistor 10 from being grounded, and an inductive reactance device 19 prevents the high-frequency current from flowing into a power supply circuit.
As for the high-frequency oscillation circuit, refer to "Design of Amplifiers and Oscillators by the S-Parameter Method", George D. Vendelin pp. 132 to 183.
It has been disclosed that such a high-frequency oscillation circuit comprising the magnetostatic wave device 1 has very high selectivity (Q) and the oscillation frequency can be changed in a wide range by changing the strength of the bias magnetic field Ho.
Further, it has also been disclosed that the above described magnetostatic wave device comprising the YIG thin film can be used from low temperature, say, -30.degree. C. because of its resonance mechanism, and a relatively inexpensive cost can be attained because the device is fabricated by the photolithography technique.
Further, a method whereby the capacitor 9 shown in FIG. 3 is omitted by separating the electrode fingers 5 each other is also disclosed in the aforementioned JP-A-63-228802.
In case the magnetostatic wave device 1 is used in the high-frequency circuit of negative resistance type as shown in FIG. 3, it is now assumed that the coupling point (a shown in FIG. 3) between the magnetostatic wave device and the negative resonance circuit is used as the measurement reference plane and the impedance of the magnetostatic wave device seen from this reference plane is .GAMMA..sub.R (with an absolute value .vertline..GAMMA..sub.R .vertline. and a phase .theta..sub.R) whereas the impedance of the negative resistance circuit seen from the reference plane is S'.sub.11 (with an absolute value .vertline.S'.sub.11 .vertline.and a phase .theta..sub.11) When a measurement is taken with a small signal level, oscillation is started under the conditions represented by the following expressions. EQU .vertline..GAMMA..sub.R .vertline. .vertline.S'.sub.11 .vertline..gtoreq.1 and .theta..sub.R +.theta..sub.11 =0
As the amplitude becomes large, the negative resistance becomes small because of nonlinearity of the transistor and other reasons. Then the relation .vertline..GAMMA..sub.R .vertline. .vertline.S'.sub.11 .vertline.=1 is satisfied and oscillation is stably continued.
In order to quickly find out the optimum condition of the frequency characteristics and the like, impedances of the magnetostatic wave device and the negative resistance circuit are measured while the circuit pattern is being subject to trimming. Possibility of oscillation can be known by comparing the reciprocal 1/S'.sub.11 of the impedance of the negative resistance circuit with the impedance .GAMMA..sub.R of the magnetostatic wave device on the Smith chart as shown in FIG. 4.
In FIG. 4, a shaded region represents an oscillation possible region seen from the negative resistance circuit. In the impedance adjustment on the side of the magnetostatic wave device for optimization of frequency characteristic, the impedance of the magnetostatic wave device must be adjusted so that the impedance of the magnetostatic wave device is brought into the shaded region shown in FIG. 4.
In reality, in case the magnetostatic wave device 1 shown in FIG. 2 is used in the high-frequency oscillation circuit shown in FIG. 3, the lower conductor plate 7 is grounded. Because of this structure, therefore, possible adjustment of the impedance on the side of the magnetostatic wave device 1 can comprise only increasing and decreasing the number of wires and length of the wire Wb connecting the pad electrode 4b to the lower conductor plate 7 and the wire Wa1 connecting the negative resistance circuit to the pad electrode 4a.
On the side of the negative resistance circuit, therefore, it was necessary to make matching with respect to the impedance of the side of the magnetostatic wave device.
Since the impedance of the negative resistance circuit side changed delicately as a result of dispersion in various constants of semiconductor amplifying devices such as transistors, dispersion in the circuit pattern, and dispersion in constants of resistors and capacitors, it was difficult to cause oscillation in the optimum state.
In some cases, therefore, it was necessary to remake the magnetostatic wave device with changed electrode dimensions.